Method and material for electrolytically brightening aluminum surfaces



' ular type.

Patented June 29, 1954 METHOD AND MATERIAL FOR ELECTRO- LYTICALLY BRIGHTENING ALUMINUM SURFACES Frederick H. Hesch, Spokane, Wash., assignor to Kaiser Aluminum & Chemical Corporation, Oakland, Calif., a corporation of Delaware No Drawing. Application July 19, 1950, Serial No. 174,799

This invention relates to bright aluminum and aluminum alloy surfaces and a method for the production thereof. More particularly the invention relates to an improved electrolyte and electrobrightening process for the treatment of aluminum and aluminum base'alloys to produce thereon durable bright surfaces of high specular reflectivity.

Highly polished or brightened aluminum surfaces are required for various decorative purposes and perhaps the major application thereof resides in the manufacture or reflectors, for the reflection of any kind of radiant energy, but particularly, for light reflection of either the diffuse or spec- Bright surfaces of the diffusely refleeting type may be produced by chemical etching, but surfaces which are highly specularly reflective more often require mechanical buffing or polishing and/or electrobrightening. Mechanical buffing alone provides a highly polished surface, but there is a distortion of the grains of the metal. This invention is primarily directed to an improved electrolyte and electrobrightening method for increasing the specular reflectivity of bright aluminum surfaces and also the preservation of such reflectivity.

Various commercial processes have been developed for electrobrightening aluminum to produce surfaces of high specular reflectivity, perhaps the most notable of which is that described in U. S. Patent 2,108,603 to R. B. Mason which comprises an electrolytic brightening step performed under certain conditions of temperature, current density and concentrations in which the aluminum article is the anode and the electrolyte, is a solution of fluoboric acid. The brightened surface is then usually given a protective anodic oxide coating in a suitable electrolyte, and such coating, if porous, is subjected to a sealing treatment to render it impervious. Other brightening electrolytes having similar characteristics are known, but less commonly used. I

In all existing methods of electrobrightening,

however, a thin soft film is formed on the brightened surface which is said to consist essentially of aluminum oxide. Such film, although at times initially transparent, due to handling or rubbing of the surface causes smudges thereon which are highly undesirable and which adversely affect to a substantial degree the reflectivity of the surface. A subsequently formed anodic oxide coating is deposited beneath the soft permeable film and when the coating is sealed, for example, by the conventional hot water treatment, the tendency for the superficial film to smudge is in- 5 Claims. (Cl. 204-33) creased. The removal of such a film, whether from the-subsequently formed oxide coating or from the brightened metal surface before anodizing, is imperative and has entailed additional and time consuming operations which have significantly increased the cost of commercial production of durable bright aluminum surfaces of high specular reflectivity. For example, in the electrobrightening process employing a hydrofluoboric acid electrolyte, the electrolytically brightened article prior to deposition of an anodic oxide coating, is subjected to a chemical dip treatment in a solvent for aluminum oxide, for example, an alkaline solvent such as a solution of an. alkali carbonate containing alkali chromate, in order to remove the smudge film, or preferably an acid. smut-removing solution, such as phosphoric and chromic acid.

It is a primary object and purpose of this invention to provide an improved electrolyte and electrolytic method by which surfaces of high specular reflectivity are imparted to aluminumv and aluminum base alloys.

A further advantage and object of the invention is the provision of an electrolyte and method for rapidly electrobrightening aluminum surfaces which overcome the difiiculties above-mentioned by eliminating the formation of a film which causes smudges, and therefore obviating the necessary additional operation ofthe prior art for removal of such smudge-forming superficial film. These and other objects and advantages will become apparent from the following detailed description of the invention. a

It has been discovered that aluminum articles may be rapidly electrolytically brightened to a high specular reflectivity without the occurrence of a superficial smudge-forming film by anodic treatment in a particular electrolyte and that such electrobrightened surfaces may have a protective anodic oxide coating subsequently applied without intermediate smudge removal treatment and without significant loss in specular reflectivity. Other forms of protective coatings may be applied to the specularly reflecting surfacesthe active constituents of the improved electrolyte are hydrofluoric, chromic and boric acids, it being understood that the hydrofluoric and boric acids may be present as hydrofluoboric acid to the extent to which they are present in combining proportions. Other boron and fluorine compounds, such as soluble salts, for example, ammonium fluoride and ammonium borate, may be substituted at least in part for the corresponding acids up to the extent to which they will form hydrofiuoboric acid, and/or hydrofluoric acid and boric acid in the presence of the quantities or concentrations of chromic acid employed in the electrolyte. In preparing the electrolyte, hydrofluoboric acid may be used instead of separate additions of hydrofluoric and boric acids, although the latter method is p efer d. Of course, the invention contemplates the use of an excess of HF over the combining proportion in regard to boric acid, since it includes compositions containing no boric acid. However, it has been found that boric acid exerts an advantageous influence either directly or by modifying the action of the other components of the electrolyte. Accordingly, in its preferred form the invention contemplates amounts of boric acid moderately in excess of combining amounts with respect to hydrofluoric, As high as five times the combining amount of boric acid has been used with excellent results.

In practicing the invention, the reflecting surface or other aluminum article is made the anode in an electrolytic cell in which the electrolyte is a solution, predominantly aqueous, comprising essentially hydrofluoric acid and chromic acid. Boric acid is a preferred electrolyte component, as indicated above. A suitable organic etching inhibitor, for example, lower molecular weight aliphatic polyhydrio alcohols, such as, glycerol, ethylene, glycol, diethylene glycol, mannitol and sorbitol may be added to the electrolyte to modify the rate of action of the active constituents. Monoethers of such polyhydric alcohols are also suitable for this purpose, such as Cellosolve and Carbitol.

Dependin upon the surfac conditions of the metal, the electrobrightening procedure may be preceded by a cleaning process to remove any dirt or grease or other foreign particles adhering to the surface. Any non-etching cleaning solvent or chemical free from objectionable attack on the metal is suitable, for example, a mild inhibited alkaline solution. Mechanical cleaning by frictional contact with the surface may be employed, but is not recommended, particularly when the surface of the work has been given a high polish of a specular nature, since any abrasive action may impair the polish thereon. For optimum specular reflectivity, the work is usually mechanically polished before electrobrightening where the shape of the article permits.

The electrobrightening process, above outlined, imparts to the surface a bright finish of high specular reflectivity, and also forms a thin transparent film which affords some protection to the bright surface against loss of reflection factor due to handling, corrosion, and the like, and which is substantially free from smudge-formin tendchoice. This may sufiice in certain applications where the reflecting surface is not subjected to excessive handling or considerable exposure to atmospheric influences.

However, where conditions of use are more adverse to preservation of the high degree of specular reflectivity of the surface, it is recommended that the electrobrightened surface be subjected to another electrolytic treatment under conditions to form thereon by anodic oxidation 2. hard, transparent oxide coating of suflicient thickness to protect the bright surface from staining, marking, corrosion or other deterioration caused by expcsure to weather, handling, washing, etc. Such anodic oxide coatings of the colorless transparent type are well known in the art and are those which consist mainly of aluminum oxide formed integral with the reflecting surface by anodic oxidation in a suitable electrolyte, such as sulfuric acid or oxalic acid. A particularly suitable electrolyte for this purpose comprising essentially sulfuric and phosphoric acids in appropriate concentrations, and the anodizing method employed is disclosed and claimed in my copendin application Serial No. 164,304 filed May 25, 1950, which provides a clear transparent non-iridescent oxide coatin causing a minimum reduction in reflection factor.

After deposition of the oxide coating, the surface may be subjected to a sealing treatment, such as the conventional hot water process, usually at a temperature of from about 160 to about 212 to render the initially porous oxide film impervious.

However, it is to be understood that such sealing procedure is not necessary to satisfactory attainment of the objects of the invention, and that bright aluminum surfaces of high specular reflectivity may be produced without such treatment. Furthermore, although it is the preferred embodiment of the invention to first preliminarily clean F the surface to free the same of grease and dirt,

then electrobrighten in the improved electrolyte, then anodically coat with oxide, and finally seal the coating, the electrolytic brightening in the solution of hydrofluoric and chromic acids, preferably also containing boric acid, may be conducted without such preliminary or subsequent steps.

Either alternating or direct current may be utilized in the operation of the electrolytic cell containing the electrolyte and wherein the aluminum article is made the anode. Direct current is preferred, however, due to the facility of control of the process afforded thereby. A cur rent density of from about 10 to amperes per square foot may be employed with good results depending upon the particular alloy being treated. Somewhat lower values may be used when the concentrations of the active electrolyte constituents, particularly HF, and the temperature are relatively low without any adverse chemical attack on the metal. A potential of about 5 to 50 volts has been found most suitable for maintaining the current densities given above, although this will vary depending upon the conductivity of the electrolyte which in turn is dependent on composition, concentration and temperature of the bath. Higher voltages are required for a low conductivity electrolyte and lower voltages suffice for those of higher conductivity. The preferred current density is adper square foot and depends on the aforementioned conditions.

The operating temperature is suitably from about 100 to about the boiling point, while the optimum or preferred temperature is from about 175 to about 190 F.

In addition to the decided advantage of smut or smudge-forming film elimination, the present invention embodies the advantage of excellent brightening results with the use of comparatively low electrolyte concentrations and current densities. Such low concentrations of electrolyte permit the use of lower current densities even at the higher temperatures employed, which latter reduce the requisite time of treatment.

A further advantage of the invention is the rapidity with which the metal surface is brightened. The time of treatment will vary, of course, with the current density, temperature, electrolyte concentrations, and the degree of brightening desired. However, aluminum surfaces of equally high spectacular reflectivity are produced in an electrolytic treatment time of only from about one and one-half to three minutes compared to a recommended five to fifteen minutes for the process involving a hydrofluoboric acid electrolyte, other conditions being equal. Moreover, no subsequent time-consuming smudge removal step is required in the present process.

Thus, the invention permits obtaining results equal to or better than the previous processes,

while greatly reducing cost due to the economy in process steps, equipment, time and power consumption.

The chromatic acid constituent of the electrolyte when combined with the hydrofluoric acid and, in addition, the preferred boric acid, present in part as hydrofluoboric acid, appears to aid in the production of a bright reflective surface and a thin clear film free of smudge-forming pro;- pensities. However, it is not intended to limit the invention to any proposed theory or mechanism, it being suflicient to state that the electrolyte and process provide a rapid and superior means of imparting to aluminum and aluminum base alloy surfaces a high specular reflectivity.

In general, it has been determined that an electrolyte comprising an aqueous solution having the following ranges of constituent concentrations, in terms of percentages by weight of solution, in any combination is highly suitable for production of bright surfaces free of smudgeforming tendencies:

The optimum concentrations or amounts by weight of the several electrolyte constituents are given below:

Percent Constituent Concentration by Wt. of

Solution Glycerol 2.5 cc./l. (95% CaHaOa). 0.3

The invention is not absolutely limited to the foregoing specific constituents, but includes such equivalent substances as will produce an electrolyte of substantially the same chemical composition among which may be mentioned the followe:

Ammonium borate, ammonium fluoride, the

alkali metal salts of boric and hydrofluoric acid,

although the presence of alkali metal ions is preferably to be avoided. Where salts of certain components are used the amount of hydrogen ion requisite for the necessary acidity must be supplied by one or more of the other constituents.

The invention is more specifically illustrated by the following example, and which is not intended as a limitation thereof:

A high purity aluminum piece, previously buffed to impart an initial specularly reflective surface was connected in an electrolytic cell as the anode. The specular or gloss reflectivity of the sample was '70 percent as measured by 2. Henry Gardner 60 Glossmeter.

The electrolyte in the cell was composed of five cubic centimeters of 48% hydrofluoric acid, 10 grams of boric acid, two grams chromic acid anhydride (chromic oxide) and 2.5 cc. of glycerol (95% CSHBOS), added to a liter of Water. The electrolyte had the following composition by weight of solution:

Per cent HF 0.3 H3503 l 1 CI'Ox 0.2 Glycerine 0.3 Water 98 2 A direct current of current density equal to 20 amperes per square foot was used at a voltage of about 35 volts for three minutes with an electrolyte temperature of about 180485 F. The surface had a specular or gloss reflectivity of 87 percent.

The brightened article was then subjected to anodic oxidation in a combined sulfuric acid, phosphoric acid electrolyte containing 3.7% and 33.6% by weight of the acids, respectively. A

direct current voltage of twenty-five volts was impressed on the cell at a current density of ten amperes per square foot with the electrolyte at a temperature of 75-80 F. for a period of ten minutes. The clear, transparent oxide film was sealed immersion in boiling water for about ten minutes. The specular reflectivity was only slightly reduced from the value for the brightened surface before deposition of the oxide film.

Although several of the aluminum base alloys when brightened by the present process do not exhibit specular reflectivity to the same degree as pure aluminum, nevertheless the electrolyte and method of the invention are fully applicable to aluminum base alloys in general.

At comparable current densities the process of the invention imparts a slightly higher gloss (mirror) reflectivity more rapidly than the process employing fluoboric acid, while eliminating essentially of from about 0.1 to 0.8% HF, from 0 to about 2.0% E3303 and from about 0.05 to 0.5% CIO3, balance substantially all water, said percentages being by weight of the total solution.

2. As an electrolyte for electrolyticallybrightening aluminum surfaces, an aqueous solution consisting essentially of from about (L1 to 0.8%

hydrofluoric acid, boric acid in amount or: to about 2% and in excess over the combir'ling weight thereof with hydrofluoric acid, and from about .05 toabout (15% chromlcacid, balance substantially all water, all percentages being by weight of total solution.

3. An electrolyte according to claim 2 which also contains from about 0.05 to 0.5% of an organic inhibitor selected from the group consisting of lower molecular weight aliphatic polyhydric alcohols and their monoethers.

4. A method of producing on aluminum and aluminum base alloys surfacesof high specular reflectivity having integral therewith a durable protective oxide coating which is clear, transparent and impermeable comprising anodically treating said aluminum surface in-an electrolyte consisting of about 0.3% hydrofluoric acid, about 1% boric acid and about 0.2% chromicacid, balance substantially all water, at .a temperature of from about 100 degrees F to about the boiling point, thereafter anodically producing thereon a transparent oxide coating, and treating said coating with hot water at a tempertuare of from about 160 degrees F. to about the boiling point to render it impervious.

As an electrolyte for electrolytically brightening aluminum and aluminum base alloy surfaces, an aqueous solution consisting essentially of from about 0.1 to 0.8% HF, from D to about 2% H3503 and from about 6.05 to 0.5% 6103, balance substantially all water, said percentages being by weight of the total solution.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,040,617 Mason et a1 May 12, 19 5 2,108,603 Mason Feb. 15, 1938 2,375,394. Tosterud May 8, 1945 OTHER REFERENCES The Iron Age, Sept. 5, 1946, page 5 

1. A METHOD OF BRIGHTENING ALUMINUM SURFACES COMPRISING SUBJECTING THE SURFACE AS ANODE TO AN ELECTROLYTIC TREATMENT IN AN ELECTROLYTE CONSISTING ESSENTIALLY OF FROM ABOUT 0.1 TO 0.8% HF, FROM 0 TO ABOUT 2.0% H3BO3 AND FROM ABOUT 0.05 TO 0.5% CRO3, BALANCE SUBSTANTIALLY ALL WATER, SAID PERCENTAGES BEING BY WEIGHT OF THE TOTAL SOLUTION. 